[1/4] kernfs: switch kernfs to use an rwsem

Commit Message

Ian Kent May 25, 2020, 5:47 a.m. UTC

The kernfs global lock restricts the ability to perform kernfs node
lookup operations in parallel.

Change the kernfs mutex to an rwsem so that, when oppertunity arises,
node searches can be done in parallel.

Signed-off-by: Ian Kent <[e-mail]>
---
 fs/kernfs/dir.c             |  119 +++++++++++++++++++++++--------------------
 fs/kernfs/file.c            |    4 +
 fs/kernfs/inode.c           |   16 +++---
 fs/kernfs/kernfs-internal.h |    5 +-
 fs/kernfs/mount.c           |   12 ++--
 fs/kernfs/symlink.c         |    4 +
 6 files changed, 86 insertions(+), 74 deletions(-)

Comments

Greg KH June 6, 2020, 6:18 p.m. UTC | #1

On Sat, Jun 06, 2020 at 11:52:16PM +0800, kernel test robot wrote:
> Greeting,
> 
> FYI, we noticed a 11827.2% improvement of stress-ng.stream.ops_per_sec due to commit:
> 
> 
> commit: ea7c5fc39ab005b501e0c7666c29db36321e4f74 ("[PATCH 1/4] kernfs: switch kernfs to use an rwsem")
> url: https://github.com/0day-ci/linux/commits/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20200525-134849
> 

Seriously?  That's a huge performance increase, and one that feels
really odd.  Why would a stress-ng test be touching sysfs?

thanks,

greg k-h

Ian Kent June 7, 2020, 1:13 a.m. UTC | #2

On Sat, 2020-06-06 at 20:18 +0200, Greg Kroah-Hartman wrote:
> On Sat, Jun 06, 2020 at 11:52:16PM +0800, kernel test robot wrote:
> > Greeting,
> > 
> > FYI, we noticed a 11827.2% improvement of stress-
> > ng.stream.ops_per_sec due to commit:
> > 
> > 
> > commit: ea7c5fc39ab005b501e0c7666c29db36321e4f74 ("[PATCH 1/4]
> > kernfs: switch kernfs to use an rwsem")
> > url: 
> > https://github.com/0day-ci/linux/commits/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20200525-134849
> > 
> 
> Seriously?  That's a huge performance increase, and one that feels
> really odd.  Why would a stress-ng test be touching sysfs?

That is unusually high even if there's a lot of sysfs or kernfs
activity and that patch shouldn't improve VFS path walk contention
very much even if it is present.

Maybe I've missed something, and the information provided doesn't
seem to be quite enough to even make a start on it.

That's going to need some analysis which, for my part, will need to
wait probably until around rc1 time frame to allow me to get through
the push down stack (reactive, postponed due to other priorities) of
jobs I have in order to get back to the fifo queue (longer term tasks,
of which this is one) list of jobs I need to do as well, ;)

Please, kernel test robot, more information about this test and what
it's doing.

Ian

Ian Kent June 7, 2020, 8:40 a.m. UTC | #3

Hi Greg,

On Mon, 2020-05-25 at 13:47 +0800, Ian Kent wrote:
> @@ -189,9 +189,9 @@ int kernfs_iop_getattr(const struct path *path,
> struct kstat *stat,
>  	struct inode *inode = d_inode(path->dentry);
>  	struct kernfs_node *kn = inode->i_private;
>  
> -	mutex_lock(&kernfs_mutex);
> +	down_read(&kernfs_rwsem);
>  	kernfs_refresh_inode(kn, inode);
> -	mutex_unlock(&kernfs_mutex);
> +	up_read(&kernfs_rwsem);
>  
>  	generic_fillattr(inode, stat);
>  	return 0;
> @@ -281,9 +281,9 @@ int kernfs_iop_permission(struct inode *inode,
> int mask)
>  
>  	kn = inode->i_private;
>  
> -	mutex_lock(&kernfs_mutex);
> +	down_read(&kernfs_rwsem);
>  	kernfs_refresh_inode(kn, inode);
> -	mutex_unlock(&kernfs_mutex);
> +	up_read(&kernfs_rwsem);
>  
>  	return generic_permission(inode, mask);
>  }

I changed these from a write lock to a read lock late in the
development.

But kernfs_refresh_inode() modifies the inode so I think I should
have taken the inode lock as well as taking the read lock.

I'll look again but a second opinion (anyone) would be welcome.

Ian

Ian Kent June 8, 2020, 9:58 a.m. UTC | #4

On Sun, 2020-06-07 at 16:40 +0800, Ian Kent wrote:
> Hi Greg,
> 
> On Mon, 2020-05-25 at 13:47 +0800, Ian Kent wrote:
> > @@ -189,9 +189,9 @@ int kernfs_iop_getattr(const struct path *path,
> > struct kstat *stat,
> >  	struct inode *inode = d_inode(path->dentry);
> >  	struct kernfs_node *kn = inode->i_private;
> >  
> > -	mutex_lock(&kernfs_mutex);
> > +	down_read(&kernfs_rwsem);
> >  	kernfs_refresh_inode(kn, inode);
> > -	mutex_unlock(&kernfs_mutex);
> > +	up_read(&kernfs_rwsem);
> >  
> >  	generic_fillattr(inode, stat);
> >  	return 0;
> > @@ -281,9 +281,9 @@ int kernfs_iop_permission(struct inode *inode,
> > int mask)
> >  
> >  	kn = inode->i_private;
> >  
> > -	mutex_lock(&kernfs_mutex);
> > +	down_read(&kernfs_rwsem);
> >  	kernfs_refresh_inode(kn, inode);
> > -	mutex_unlock(&kernfs_mutex);
> > +	up_read(&kernfs_rwsem);
> >  
> >  	return generic_permission(inode, mask);
> >  }
> 
> I changed these from a write lock to a read lock late in the
> development.
> 
> But kernfs_refresh_inode() modifies the inode so I think I should
> have taken the inode lock as well as taking the read lock.
> 
> I'll look again but a second opinion (anyone) would be welcome.

I had a look at this today and came up with a couple of patches
to fix it, I don't particularly like to have to do what I did
but I don't think there's any other choice. That's because the
rb tree locking is under significant contention and changing
this back to use the write lock will adversely affect that.

But unless I can find out more about the anomalous kernel test
robot result I can't do anything!

Providing a job.yaml to reproduce it with the hardware specification
of the lkp machine it was run on and no guidelines on what that test
does and what the test needs so it can actually be reproduced isn't
that useful.

Ian

Chen, Rong A June 11, 2020, 2:06 a.m. UTC | #5

On Sun, Jun 07, 2020 at 09:13:08AM +0800, Ian Kent wrote:
> On Sat, 2020-06-06 at 20:18 +0200, Greg Kroah-Hartman wrote:
> > On Sat, Jun 06, 2020 at 11:52:16PM +0800, kernel test robot wrote:
> > > Greeting,
> > > 
> > > FYI, we noticed a 11827.2% improvement of stress-
> > > ng.stream.ops_per_sec due to commit:
> > > 
> > > 
> > > commit: ea7c5fc39ab005b501e0c7666c29db36321e4f74 ("[PATCH 1/4]
> > > kernfs: switch kernfs to use an rwsem")
> > > url: 
> > > https://github.com/0day-ci/linux/commits/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20200525-134849
> > > 
> > 
> > Seriously?  That's a huge performance increase, and one that feels
> > really odd.  Why would a stress-ng test be touching sysfs?
> 
> That is unusually high even if there's a lot of sysfs or kernfs
> activity and that patch shouldn't improve VFS path walk contention
> very much even if it is present.
> 
> Maybe I've missed something, and the information provided doesn't
> seem to be quite enough to even make a start on it.
> 
> That's going to need some analysis which, for my part, will need to
> wait probably until around rc1 time frame to allow me to get through
> the push down stack (reactive, postponed due to other priorities) of
> jobs I have in order to get back to the fifo queue (longer term tasks,
> of which this is one) list of jobs I need to do as well, ;)
> 
> Please, kernel test robot, more information about this test and what
> it's doing.
> 

Hi Ian,

We increased the timeout of stress-ng from 1s to 32s, and there's only
3% improvement of stress-ng.stream.ops_per_sec:

fefcfc968723caf9  ea7c5fc39ab005b501e0c7666c  testcase/testparams/testbox
----------------  --------------------------  ---------------------------
         %stddev      change         %stddev
             \          |                \  
     10686               3%      11037        stress-ng/cpu-cache-performance-1HDD-100%-32s-ucode=0x500002c/lkp-csl-2sp5
     10686               3%      11037        GEO-MEAN stress-ng.stream.ops_per_sec

It seems the result of stress-ng is inaccurate if test time too
short, we'll increase the test time to avoid unreasonable results,
sorry for the inconvenience.

Best Regards,
Rong Chen

Rick Lindsley June 11, 2020, 2:20 a.m. UTC | #6

On 6/10/20 7:06 PM, kernel test robot wrote:
> On Sun, Jun 07, 2020 at 09:13:08AM +0800, Ian Kent wrote:
>
> It seems the result of stress-ng is inaccurate if test time too
> short, we'll increase the test time to avoid unreasonable results,
> sorry for the inconvenience.

Thank you for your response!  I was examining the 25 tests in the 'cpu-cache' class and had nothing but head scratching so far on what could be having that effect.

Rick

Ian Kent June 11, 2020, 3:02 a.m. UTC | #7

On Thu, 2020-06-11 at 10:06 +0800, kernel test robot wrote:
> On Sun, Jun 07, 2020 at 09:13:08AM +0800, Ian Kent wrote:
> > On Sat, 2020-06-06 at 20:18 +0200, Greg Kroah-Hartman wrote:
> > > On Sat, Jun 06, 2020 at 11:52:16PM +0800, kernel test robot
> > > wrote:
> > > > Greeting,
> > > > 
> > > > FYI, we noticed a 11827.2% improvement of stress-
> > > > ng.stream.ops_per_sec due to commit:
> > > > 
> > > > 
> > > > commit: ea7c5fc39ab005b501e0c7666c29db36321e4f74 ("[PATCH 1/4]
> > > > kernfs: switch kernfs to use an rwsem")
> > > > url: 
> > > > https://github.com/0day-ci/linux/commits/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20200525-134849
> > > > 
> > > 
> > > Seriously?  That's a huge performance increase, and one that
> > > feels
> > > really odd.  Why would a stress-ng test be touching sysfs?
> > 
> > That is unusually high even if there's a lot of sysfs or kernfs
> > activity and that patch shouldn't improve VFS path walk contention
> > very much even if it is present.
> > 
> > Maybe I've missed something, and the information provided doesn't
> > seem to be quite enough to even make a start on it.
> > 
> > That's going to need some analysis which, for my part, will need to
> > wait probably until around rc1 time frame to allow me to get
> > through
> > the push down stack (reactive, postponed due to other priorities)
> > of
> > jobs I have in order to get back to the fifo queue (longer term
> > tasks,
> > of which this is one) list of jobs I need to do as well, ;)
> > 
> > Please, kernel test robot, more information about this test and
> > what
> > it's doing.
> > 
> 
> Hi Ian,
> 
> We increased the timeout of stress-ng from 1s to 32s, and there's
> only
> 3% improvement of stress-ng.stream.ops_per_sec:
> 
> fefcfc968723caf9  ea7c5fc39ab005b501e0c7666c  testcase/testparams/tes
> tbox
> ----------------  --------------------------  -----------------------
> ----
>          %stddev      change         %stddev
>              \          |                \  
>      10686               3%      11037        stress-ng/cpu-cache-
> performance-1HDD-100%-32s-ucode=0x500002c/lkp-csl-2sp5
>      10686               3%      11037        GEO-MEAN stress-
> ng.stream.ops_per_sec
> 
> It seems the result of stress-ng is inaccurate if test time too
> short, we'll increase the test time to avoid unreasonable results,
> sorry for the inconvenience.

Haha, I was worried there wasn't anything that could be done to
work out what was wrong.

I had tried to reproduce it, and failed since the job file specifies
a host config that I simply don't have, and I don't get how to alter
the job to suit, or how to specify a host definition file.

I also couldn't work out what parameters where used in running the
test so I was about to ask on the lkp list after working through
this in a VM.

So your timing on looking into this is fortunate, for sure.
Thank you very much for that.

Now, Greg, there's that locking I changed around kernfs_refresh_inode()
that I need to fix which I re-considered as a result of this, so that's
a plus for the testing because it's certainly wrong.

I'll have another look at that and boot test it on a couple of systems
then post a v2 for you to consider. What I've done might offend your
sensibilities as it does mine, or perhaps not so much.

Ian

Patch

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 9aec80b9d7c6..d8213fc65eba 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -17,7 +17,7 @@ 
 
 #include "kernfs-internal.h"
 
-DEFINE_MUTEX(kernfs_mutex);
+DECLARE_RWSEM(kernfs_rwsem);
 static DEFINE_SPINLOCK(kernfs_rename_lock);	/* kn->parent and ->name */
 static char kernfs_pr_cont_buf[PATH_MAX];	/* protected by rename_lock */
 static DEFINE_SPINLOCK(kernfs_idr_lock);	/* root->ino_idr */
@@ -26,10 +26,21 @@  static DEFINE_SPINLOCK(kernfs_idr_lock);	/* root->ino_idr */
 
 static bool kernfs_active(struct kernfs_node *kn)
 {
-	lockdep_assert_held(&kernfs_mutex);
 	return atomic_read(&kn->active) >= 0;
 }
 
+static bool kernfs_active_write(struct kernfs_node *kn)
+{
+	lockdep_assert_held_write(&kernfs_rwsem);
+	return kernfs_active(kn);
+}
+
+static bool kernfs_active_read(struct kernfs_node *kn)
+{
+	lockdep_assert_held_read(&kernfs_rwsem);
+	return kernfs_active(kn);
+}
+
 static bool kernfs_lockdep(struct kernfs_node *kn)
 {
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -340,7 +351,7 @@  static int kernfs_sd_compare(const struct kernfs_node *left,
  *	@kn->parent->dir.children.
  *
  *	Locking:
- *	mutex_lock(kernfs_mutex)
+ *	kernfs_rwsem write lock
  *
  *	RETURNS:
  *	0 on susccess -EEXIST on failure.
@@ -385,7 +396,7 @@  static int kernfs_link_sibling(struct kernfs_node *kn)
  *	removed, %false if @kn wasn't on the rbtree.
  *
  *	Locking:
- *	mutex_lock(kernfs_mutex)
+ *	kernfs_rwsem write lock
  */
 static bool kernfs_unlink_sibling(struct kernfs_node *kn)
 {
@@ -455,14 +466,14 @@  void kernfs_put_active(struct kernfs_node *kn)
  * return after draining is complete.
  */
 static void kernfs_drain(struct kernfs_node *kn)
-	__releases(&kernfs_mutex) __acquires(&kernfs_mutex)
+	__releases(&kernfs_rwsem) __acquires(&kernfs_rwsem)
 {
 	struct kernfs_root *root = kernfs_root(kn);
 
-	lockdep_assert_held(&kernfs_mutex);
+	lockdep_assert_held_write(&kernfs_rwsem);
 	WARN_ON_ONCE(kernfs_active(kn));
 
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 
 	if (kernfs_lockdep(kn)) {
 		rwsem_acquire(&kn->dep_map, 0, 0, _RET_IP_);
@@ -481,7 +492,7 @@  static void kernfs_drain(struct kernfs_node *kn)
 
 	kernfs_drain_open_files(kn);
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 }
 
 /**
@@ -560,10 +571,10 @@  static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 		goto out_bad_unlocked;
 
 	kn = kernfs_dentry_node(dentry);
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_rwsem);
 
 	/* The kernfs node has been deactivated */
-	if (!kernfs_active(kn))
+	if (!kernfs_active_read(kn))
 		goto out_bad;
 
 	/* The kernfs node has been moved? */
@@ -579,10 +590,10 @@  static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 	    kernfs_info(dentry->d_sb)->ns != kn->ns)
 		goto out_bad;
 
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 	return 1;
 out_bad:
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 out_bad_unlocked:
 	return 0;
 }
@@ -764,7 +775,7 @@  int kernfs_add_one(struct kernfs_node *kn)
 	bool has_ns;
 	int ret;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 
 	ret = -EINVAL;
 	has_ns = kernfs_ns_enabled(parent);
@@ -779,7 +790,7 @@  int kernfs_add_one(struct kernfs_node *kn)
 	if (parent->flags & KERNFS_EMPTY_DIR)
 		goto out_unlock;
 
-	if ((parent->flags & KERNFS_ACTIVATED) && !kernfs_active(parent))
+	if ((parent->flags & KERNFS_ACTIVATED) && !kernfs_active_write(parent))
 		goto out_unlock;
 
 	kn->hash = kernfs_name_hash(kn->name, kn->ns);
@@ -795,7 +806,7 @@  int kernfs_add_one(struct kernfs_node *kn)
 		ps_iattr->ia_mtime = ps_iattr->ia_ctime;
 	}
 
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 
 	/*
 	 * Activate the new node unless CREATE_DEACTIVATED is requested.
@@ -809,7 +820,7 @@  int kernfs_add_one(struct kernfs_node *kn)
 	return 0;
 
 out_unlock:
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 	return ret;
 }
 
@@ -830,7 +841,7 @@  static struct kernfs_node *kernfs_find_ns(struct kernfs_node *parent,
 	bool has_ns = kernfs_ns_enabled(parent);
 	unsigned int hash;
 
-	lockdep_assert_held(&kernfs_mutex);
+	lockdep_assert_held(&kernfs_rwsem);
 
 	if (has_ns != (bool)ns) {
 		WARN(1, KERN_WARNING "kernfs: ns %s in '%s' for '%s'\n",
@@ -862,7 +873,7 @@  static struct kernfs_node *kernfs_walk_ns(struct kernfs_node *parent,
 	size_t len;
 	char *p, *name;
 
-	lockdep_assert_held(&kernfs_mutex);
+	lockdep_assert_held_read(&kernfs_rwsem);
 
 	/* grab kernfs_rename_lock to piggy back on kernfs_pr_cont_buf */
 	spin_lock_irq(&kernfs_rename_lock);
@@ -902,10 +913,10 @@  struct kernfs_node *kernfs_find_and_get_ns(struct kernfs_node *parent,
 {
 	struct kernfs_node *kn;
 
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_rwsem);
 	kn = kernfs_find_ns(parent, name, ns);
 	kernfs_get(kn);
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 
 	return kn;
 }
@@ -926,10 +937,10 @@  struct kernfs_node *kernfs_walk_and_get_ns(struct kernfs_node *parent,
 {
 	struct kernfs_node *kn;
 
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_rwsem);
 	kn = kernfs_walk_ns(parent, path, ns);
 	kernfs_get(kn);
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 
 	return kn;
 }
@@ -1084,7 +1095,7 @@  static struct dentry *kernfs_iop_lookup(struct inode *dir,
 	struct inode *inode;
 	const void *ns = NULL;
 
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_rwsem);
 
 	if (kernfs_ns_enabled(parent))
 		ns = kernfs_info(dir->i_sb)->ns;
@@ -1107,7 +1118,7 @@  static struct dentry *kernfs_iop_lookup(struct inode *dir,
 	/* instantiate and hash dentry */
 	ret = d_splice_alias(inode, dentry);
  out_unlock:
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 	return ret;
 }
 
@@ -1226,7 +1237,7 @@  static struct kernfs_node *kernfs_next_descendant_post(struct kernfs_node *pos,
 {
 	struct rb_node *rbn;
 
-	lockdep_assert_held(&kernfs_mutex);
+	lockdep_assert_held_write(&kernfs_rwsem);
 
 	/* if first iteration, visit leftmost descendant which may be root */
 	if (!pos)
@@ -1262,7 +1273,7 @@  void kernfs_activate(struct kernfs_node *kn)
 {
 	struct kernfs_node *pos;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 
 	pos = NULL;
 	while ((pos = kernfs_next_descendant_post(pos, kn))) {
@@ -1276,14 +1287,14 @@  void kernfs_activate(struct kernfs_node *kn)
 		pos->flags |= KERNFS_ACTIVATED;
 	}
 
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 }
 
 static void __kernfs_remove(struct kernfs_node *kn)
 {
 	struct kernfs_node *pos;
 
-	lockdep_assert_held(&kernfs_mutex);
+	lockdep_assert_held_write(&kernfs_rwsem);
 
 	/*
 	 * Short-circuit if non-root @kn has already finished removal.
@@ -1298,7 +1309,7 @@  static void __kernfs_remove(struct kernfs_node *kn)
 	/* prevent any new usage under @kn by deactivating all nodes */
 	pos = NULL;
 	while ((pos = kernfs_next_descendant_post(pos, kn)))
-		if (kernfs_active(pos))
+		if (kernfs_active_write(pos))
 			atomic_add(KN_DEACTIVATED_BIAS, &pos->active);
 
 	/* deactivate and unlink the subtree node-by-node */
@@ -1306,7 +1317,7 @@  static void __kernfs_remove(struct kernfs_node *kn)
 		pos = kernfs_leftmost_descendant(kn);
 
 		/*
-		 * kernfs_drain() drops kernfs_mutex temporarily and @pos's
+		 * kernfs_drain() drops kernfs_rwsem temporarily and @pos's
 		 * base ref could have been put by someone else by the time
 		 * the function returns.  Make sure it doesn't go away
 		 * underneath us.
@@ -1353,9 +1364,9 @@  static void __kernfs_remove(struct kernfs_node *kn)
  */
 void kernfs_remove(struct kernfs_node *kn)
 {
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 	__kernfs_remove(kn);
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 }
 
 /**
@@ -1442,17 +1453,17 @@  bool kernfs_remove_self(struct kernfs_node *kn)
 {
 	bool ret;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 	kernfs_break_active_protection(kn);
 
 	/*
 	 * SUICIDAL is used to arbitrate among competing invocations.  Only
 	 * the first one will actually perform removal.  When the removal
 	 * is complete, SUICIDED is set and the active ref is restored
-	 * while holding kernfs_mutex.  The ones which lost arbitration
-	 * waits for SUICDED && drained which can happen only after the
-	 * enclosing kernfs operation which executed the winning instance
-	 * of kernfs_remove_self() finished.
+	 * while holding kernfs_rwsem for write.  The ones which lost
+	 * arbitration waits for SUICIDED && drained which can happen only
+	 * after the enclosing kernfs operation which executed the winning
+	 * instance of kernfs_remove_self() finished.
 	 */
 	if (!(kn->flags & KERNFS_SUICIDAL)) {
 		kn->flags |= KERNFS_SUICIDAL;
@@ -1470,9 +1481,9 @@  bool kernfs_remove_self(struct kernfs_node *kn)
 			    atomic_read(&kn->active) == KN_DEACTIVATED_BIAS)
 				break;
 
-			mutex_unlock(&kernfs_mutex);
+			up_write(&kernfs_rwsem);
 			schedule();
-			mutex_lock(&kernfs_mutex);
+			down_write(&kernfs_rwsem);
 		}
 		finish_wait(waitq, &wait);
 		WARN_ON_ONCE(!RB_EMPTY_NODE(&kn->rb));
@@ -1480,12 +1491,12 @@  bool kernfs_remove_self(struct kernfs_node *kn)
 	}
 
 	/*
-	 * This must be done while holding kernfs_mutex; otherwise, waiting
-	 * for SUICIDED && deactivated could finish prematurely.
+	 * This must be done while holding kernfs_rwsem for write; otherwise,
+	 * waiting for SUICIDED && deactivated could finish prematurely.
 	 */
 	kernfs_unbreak_active_protection(kn);
 
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 	return ret;
 }
 
@@ -1509,13 +1520,13 @@  int kernfs_remove_by_name_ns(struct kernfs_node *parent, const char *name,
 		return -ENOENT;
 	}
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 
 	kn = kernfs_find_ns(parent, name, ns);
 	if (kn)
 		__kernfs_remove(kn);
 
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 
 	if (kn)
 		return 0;
@@ -1541,10 +1552,10 @@  int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 	if (!kn->parent)
 		return -EINVAL;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 
 	error = -ENOENT;
-	if (!kernfs_active(kn) || !kernfs_active(new_parent) ||
+	if (!kernfs_active_write(kn) || !kernfs_active_write(new_parent) ||
 	    (new_parent->flags & KERNFS_EMPTY_DIR))
 		goto out;
 
@@ -1595,7 +1606,7 @@  int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 
 	error = 0;
  out:
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 	return error;
 }
 
@@ -1615,7 +1626,7 @@  static struct kernfs_node *kernfs_dir_pos(const void *ns,
 	struct kernfs_node *parent, loff_t hash, struct kernfs_node *pos)
 {
 	if (pos) {
-		int valid = kernfs_active(pos) &&
+		int valid = kernfs_active_read(pos) &&
 			pos->parent == parent && hash == pos->hash;
 		kernfs_put(pos);
 		if (!valid)
@@ -1635,7 +1646,7 @@  static struct kernfs_node *kernfs_dir_pos(const void *ns,
 		}
 	}
 	/* Skip over entries which are dying/dead or in the wrong namespace */
-	while (pos && (!kernfs_active(pos) || pos->ns != ns)) {
+	while (pos && (!kernfs_active_read(pos) || pos->ns != ns)) {
 		struct rb_node *node = rb_next(&pos->rb);
 		if (!node)
 			pos = NULL;
@@ -1656,7 +1667,7 @@  static struct kernfs_node *kernfs_dir_next_pos(const void *ns,
 				pos = NULL;
 			else
 				pos = rb_to_kn(node);
-		} while (pos && (!kernfs_active(pos) || pos->ns != ns));
+		} while (pos && (!kernfs_active_read(pos) || pos->ns != ns));
 	}
 	return pos;
 }
@@ -1670,7 +1681,7 @@  static int kernfs_fop_readdir(struct file *file, struct dir_context *ctx)
 
 	if (!dir_emit_dots(file, ctx))
 		return 0;
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_rwsem);
 
 	if (kernfs_ns_enabled(parent))
 		ns = kernfs_info(dentry->d_sb)->ns;
@@ -1687,12 +1698,12 @@  static int kernfs_fop_readdir(struct file *file, struct dir_context *ctx)
 		file->private_data = pos;
 		kernfs_get(pos);
 
-		mutex_unlock(&kernfs_mutex);
+		up_read(&kernfs_rwsem);
 		if (!dir_emit(ctx, name, len, ino, type))
 			return 0;
-		mutex_lock(&kernfs_mutex);
+		down_read(&kernfs_rwsem);
 	}
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 	file->private_data = NULL;
 	ctx->pos = INT_MAX;
 	return 0;
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 34366db3620d..455caea6ab0b 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -879,7 +879,7 @@  static void kernfs_notify_workfn(struct work_struct *work)
 	spin_unlock_irq(&kernfs_notify_lock);
 
 	/* kick fsnotify */
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 
 	list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
 		struct kernfs_node *parent;
@@ -916,7 +916,7 @@  static void kernfs_notify_workfn(struct work_struct *work)
 		iput(inode);
 	}
 
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 	kernfs_put(kn);
 	goto repeat;
 }
diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index fc2469a20fed..32e334e0d687 100644
--- a/fs/kernfs/inode.c
+++ b/fs/kernfs/inode.c
@@ -106,9 +106,9 @@  int kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)
 {
 	int ret;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 	ret = __kernfs_setattr(kn, iattr);
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 	return ret;
 }
 
@@ -121,7 +121,7 @@  int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr)
 	if (!kn)
 		return -EINVAL;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 	error = setattr_prepare(dentry, iattr);
 	if (error)
 		goto out;
@@ -134,7 +134,7 @@  int kernfs_iop_setattr(struct dentry *dentry, struct iattr *iattr)
 	setattr_copy(inode, iattr);
 
 out:
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 	return error;
 }
 
@@ -189,9 +189,9 @@  int kernfs_iop_getattr(const struct path *path, struct kstat *stat,
 	struct inode *inode = d_inode(path->dentry);
 	struct kernfs_node *kn = inode->i_private;
 
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_rwsem);
 	kernfs_refresh_inode(kn, inode);
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 
 	generic_fillattr(inode, stat);
 	return 0;
@@ -281,9 +281,9 @@  int kernfs_iop_permission(struct inode *inode, int mask)
 
 	kn = inode->i_private;
 
-	mutex_lock(&kernfs_mutex);
+	down_read(&kernfs_rwsem);
 	kernfs_refresh_inode(kn, inode);
-	mutex_unlock(&kernfs_mutex);
+	up_read(&kernfs_rwsem);
 
 	return generic_permission(inode, mask);
 }
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 7ee97ef59184..097c1a989aa4 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -13,6 +13,7 @@ 
 #include <linux/lockdep.h>
 #include <linux/fs.h>
 #include <linux/mutex.h>
+#include <linux/rwsem.h>
 #include <linux/xattr.h>
 
 #include <linux/kernfs.h>
@@ -69,7 +70,7 @@  struct kernfs_super_info {
 	 */
 	const void		*ns;
 
-	/* anchored at kernfs_root->supers, protected by kernfs_mutex */
+	/* anchored at kernfs_root->supers, protected by kernfs_rwsem */
 	struct list_head	node;
 };
 #define kernfs_info(SB) ((struct kernfs_super_info *)(SB->s_fs_info))
@@ -99,7 +100,7 @@  int __kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr);
 /*
  * dir.c
  */
-extern struct mutex kernfs_mutex;
+extern struct rw_semaphore kernfs_rwsem;
 extern const struct dentry_operations kernfs_dops;
 extern const struct file_operations kernfs_dir_fops;
 extern const struct inode_operations kernfs_dir_iops;
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 9dc7e7a64e10..baa4155ba2ed 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -255,9 +255,9 @@  static int kernfs_fill_super(struct super_block *sb, struct kernfs_fs_context *k
 	sb->s_shrink.seeks = 0;
 
 	/* get root inode, initialize and unlock it */
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 	inode = kernfs_get_inode(sb, info->root->kn);
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 	if (!inode) {
 		pr_debug("kernfs: could not get root inode\n");
 		return -ENOMEM;
@@ -344,9 +344,9 @@  int kernfs_get_tree(struct fs_context *fc)
 		}
 		sb->s_flags |= SB_ACTIVE;
 
-		mutex_lock(&kernfs_mutex);
+		down_write(&kernfs_rwsem);
 		list_add(&info->node, &info->root->supers);
-		mutex_unlock(&kernfs_mutex);
+		up_write(&kernfs_rwsem);
 	}
 
 	fc->root = dget(sb->s_root);
@@ -372,9 +372,9 @@  void kernfs_kill_sb(struct super_block *sb)
 {
 	struct kernfs_super_info *info = kernfs_info(sb);
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 	list_del(&info->node);
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 
 	/*
 	 * Remove the superblock from fs_supers/s_instances
diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
index 5432883d819f..7246b470de3c 100644
--- a/fs/kernfs/symlink.c
+++ b/fs/kernfs/symlink.c
@@ -116,9 +116,9 @@  static int kernfs_getlink(struct inode *inode, char *path)
 	struct kernfs_node *target = kn->symlink.target_kn;
 	int error;
 
-	mutex_lock(&kernfs_mutex);
+	down_write(&kernfs_rwsem);
 	error = kernfs_get_target_path(parent, target, path);
-	mutex_unlock(&kernfs_mutex);
+	up_write(&kernfs_rwsem);
 
 	return error;
 }